This invention relates to a process for removing phenol from waste waters and in particular to a method for removing phenol from waste waters that occur in the manufacture of phenol by the cumene process wherein cumene is used as an extractant.
In the course of time numerous processes for removing phenol from waste waters have become known. Most of these processes have to do with the dephenolization of waste waters such as those produced in coke plants. In almost every case the phenol-containing waste waters are treated with extractants, the phenol being absorbed by the extractant. The phenol may be recovered from these phenol-laden extractants in a number of ways.
Processes have been developed in which the solvent is distilled from the phenol provided that these solvents have a lower boiling point than phenol. On the other hand, processes have also become known in which the phenol is separated from the extractant by distillation, that is, in cases in which the extractant has a higher boiling point than phenol.
Very frequently methods are used for recovering the phenol from the extractant in which the phenol is washed out with alkalies, especially soda lye.
In the manufacture of phenol by the cumene process, waste waters are produced which differ considerably in their composition from those produced in the processing of tars. It may be said that the compositions of these waste waters differ basically from one another, and in general the only contaminant component they have in common is phenol.
Accordingly, new methods have had to be developed for treating the waste waters which are produced in the manufacture of phenol by the cumene process. Here, again, the approach has been to extract these waste waters with solvents, and benzene, toluene, xylene, cumene, acetophenone, mesityl oxide, butyl acetate, tricresyl phosphate and other substances have been proposed. Furthermore, mixtures, such as mesityl oxide with benzene, toluene, xylene or cumene, have been represented as especially effective. Likewise, acetophenone mixed with other aromatics has been described as especially effective.
In addition to the use of pure organic extractants and mixtures of extractants, it has been proposed to reduce the pH of the waste waters, because it is said that in the acid ranges the dephenolization can be performed to a particularly complete degree. pH values especially ranging from 2 to 4 have been named. In addition, sodium phenolate solutions, which are to have a very constant content of free soda lye, have simultaneously been used for the removal of the phenol from the extractants.
Owing to the great number of processes proposed for the dephenolization of waste waters and for the removal of the phenol from the extractants, it is apparent that most of these processes are not satisfactory and either involve difficulty in their technical application or are not at all practical.
For example, the technical practice of the method described in German Pat. 1,075,119 has proven to be inordinately expensive because the waste waters to be dephenolized have to be greatly acidified. In general, a pH value of 1, or usually even less, has to be established in order to permit the trouble-free extraction of the phenol with cumene. In the case of higher pH values, strata develop during the extraction, in which water and cumene, contaminated with components of the waste water, form a kind of emulsion which cannot be broken down at pH values above 1, and which not only encumber the procedure but make the dephenolization virtually impossible.
These emulsion layers, hereinafter to be called "floc layers," become concentrated, and as a result waste water containing cumene, on the one hand, and cumene containing waste water, on the other hand, are discharged from the separating tank. The waste water containing cumene still contains appreciable amounts of the phenol, and the cumene containing waste water leads to considerable trouble in the cumene washing operation that follows, in which soda lye is involved.
Only through very great acidification of the waste water has it been possible in any way to remedy these difficulties. The consequence of this necessary measure has been an inordinately great consumption of sulfuric acid which is used for the acidification. Before this very acid waste water can be discharged into the sewage system after the dephenolization it is necessary to neutralize it, and this results in correspondingly high consumption of soda lye. As a consequence of this procedure there is a high salt content in the waste water.
On account of the great acidification of the waste water being dephenolized, the entire dephenolization apparatus has to be made of high grade steel.
As a result of the measures described above, and of the expense which they entail, the earnings from the phenolate lye or from the phenol produced therefrom fall far short of covering the costs involved.
Numerous series of experiments attempting to eliminate the separating difficulties have shown that in the extraction of waste waters of the kind produced in the manufacture of phenol by the cumene process, not only phenol but numerous other impurities find their way into the extractant. Of these impurities the "floc forming" substances are exceedingly troublesome. In the lye washing that follows not only phenol but also these floc forming substances are absorbed by the lye and after the phenolate lye is broken down they get back into the waste water being dephenolized. The floc forming substances are concentrated by this necessary recirculation and cause the above-described difficulties.